1. Field of the Invention
The present invention relates to a surface potential detection apparatus and a surface potential detection method in which a surface potential is measured through a non-contact method.
2. Discussion of Background
A surface potential detection apparatus of this type, which may be employed to detect the surface potential of a photosensitive drum in a photocopier, laser beam printer or the like through a non-contact method, for instance, utilizes a surface potential sensor that generates an AC signal corresponding to the surface potential of the photosensitive drum by using a tuning fork to mechanically interrupt the electrical field between a detection electrode and the photosensitive drum as disclosed in Japanese Examined Patent Publication No. 6467/1991 and the like. The AC signal is then amplified at a pre-amplifier and the amplified signal is guided to a synchronous detection circuit via an isolator, where the signal wave is detected with a signal that is synchronized with the mechanical interruption. The synchronous detection output signal output from the synchronous detection circuit is converted to a DC signal at an integrating circuit. The DC signal obtained at the integrating circuit is input to a high-voltage amplifier.
The individual circuits share a common ground line. The high-voltage amplifier controls the potential at the common ground line in conformance to the input DC signal so as to equalize the potential at the common ground line to the potential at the surface of the photosensitive drum which is the measurement target surface. By extracting the potential at the common ground line via an attenuator, a buffer amplifier and the like, a surface potential signal is obtained. The common ground line achieves a floating relationship with a ground potential or the frame ground potential.
The greatest advantage achieved through this method is that even when the distance between the surface potential sensor that includes the tuning fork and the detection electrode and the surface of the photosensitive drum constituting the measurement target surface changes, a highly accurate surface potential detection signal manifesting a very low degree of distance dependency is obtained.
The surface potential detection apparatus described above normally includes two units, i.e., a surface potential sensor and a signal processing device. The surface potential sensor is constituted as a type of probe which includes a detection electrode, a tuning fork, a drive circuit, a pre-amplifier and the like. The signal processing device includes the remaining circuit portion that is needed to achieve the function as a surface potential detection apparatus.
Numerous tandem type high-speed photocopiers and laser printers have been proposed and put into practical use in recent years. In these image generating apparatuses utilizing four photosensitive drums (cyan, magenta, yellow and black), the surface potentials at the four photosensitive drums must be measured.
Since it is necessary to provide one surface potential sensor and one signal processing device in conjunction with each photosensitive drum in the prior art, four sets each having a surface potential sensor and a signal processing device must be provided for the four photosensitive drums. Each signal processing device constituting a surface potential detection apparatus has a complicated circuit structure and includes two transformers, which is bound to cause increases in the size, the weight and the production cost of the device. Accordingly, in the prior art, the size, the weight and the production cost of the surface potential detection apparatus itself are notably significant since the signal processing device must be provided for each of the four photosensitive drums, posing a critical problem to be addressed.
In addition, since the surface potential sensor in the prior art, which obtains an AC signal corresponding to the surface potential at the photosensitive drum by mechanically interrupting the electrical field between the detection electrode and the photosensitive drum with a tuning fork, has a complicated structure that includes a mechanically movable part, a great deal of time and work must be expended to manufacture, assemble, adjust, maintain and inspect the surface potential sensor.
It is a first object of the present invention to provide a surface potential detection apparatus that achieves simplification in the circuit, miniaturization and reductions in the weight and the production cost.
It is a second object of the present invention to provide a new and improved surface potential detection method which makes it possible to detect a surface potential with a high degree of reliability through a simple structure and a surface potential detection apparatus and a surface potential detection sensor that may be directly utilized to implement the method.
It is a third object of the present invention to provide a new and improved surface potential detection method which greatly facilitates the processes of production, assembly, adjustment, inspection, maintenance and the like and a surface potential detection apparatus and a surface potential detection sensor that may be directly utilized to implement the method.
In order to achieve the first object, the surface potential detection apparatus according to the present invention includes a plurality of surface potential sensors, a switching circuit and a signal processing circuit. The plurality of surface potential sensors are provided independently of one another. The switching circuit individually selects and outputs signals provided by the surface potential sensors with varied timing for each surface potential sensor. The signal processing circuit is connected to the plurality of surface potential sensors via the switching circuit to serve the plurality of surface potential sensors.
Since the surface potential detection apparatus according to the present invention is provided with the plurality of surface potential sensors which function independently of one another, the surface potential sensors may be utilized in conjunction with four photosensitive drums (cyan, magenta, yellow and black) in an image generating apparatus such as a tandem type high-speed photocopier or a tandem type high-speed laser beam printer to individually detect the surface potentials at the four photosensitive drums.
The switching circuit individually selects and outputs the signals provided by the plurality of surface potential sensors with varied timing for each surface potential sensor. Thus, the signals that are output from the plurality of surface potential sensors can be extracted separately from one another over time.
Since the signal processing circuit is connected to the plurality of surface potential sensors via the switching circuit, it receives the signals from the surface potential sensors separately over time. Then, it implements necessary signal processing within time frames allocated to the individual surface potential sensors.
The signal processing circuit is shared by the plurality of surface potential sensors. Thus, only a single signal processing circuit is required. This greatly simplifies the circuit structure and also greatly reduces the size, the weight and the production cost compared to the surface potential detection apparatus in the prior art which necessitates four sets each constituted of a surface potential sensor and a signal processing device to be provided in conjunction with the four photosensitive drums.
Each of the plurality of surface potential sensors may assume a structure adopted in the known art. Typically, the surface potential sensors each generate an AC signal corresponding to the surface potential at the measurement target surface by interrupting the electrical field between the detection electrode and the measurement target surface. In this case, the switching circuit individually selects and outputs the detection signal and a synchronous signal which is in synchronization with a drive signal for interruption provided by each surface potential sensor with varying timings for the individual surface potential sensors.
In a typical structure assumed by the surface potential detection apparatus according to the present invention, the plurality of surface potential sensors and the signal processing circuit share a common ground line. The signal processing circuit implements control on the potential at the common ground line so as to roughly equalize the potential at the common ground line to the potential at a measurement target surface. As a result, a highly accurate surface potential detection signal with a very low degree of distance dependency is obtained even when the distance between a surface potential sensor and the surface of the corresponding photosensitive drum constituting the measurement target surface changes.
In order to achieve the second and third objects, a stationary electrode is provided between a surface potential measurement target and a detection electrode, the areas surrounding the detection electrode and the stationary electrode are electromagnetically shielded and the stationary electrode is set facing opposite the surface potential measurement target through an unshielded detection window to enable surface potential detection according to the present invention. The impedance between the shield potential and the stationary electrode is cyclically changed.
By providing the stationary electrode between the surface potential measurement target and the detection electrode, setting the stationary electrode facing opposite the surface potential measurement target through the detection window and changing the impedance between the shield potential and the stationary electrode as described above, the electrical field between the detection electrode and the surface potential measurement target is also allowed to change cyclically in correspondence to the cyclical change in the impedance. As a result, a signal (an AC signal) corresponding to the cyclical change in the electrical field between the detection electrode and the surface potential measurement target is generated at the detection electrode. Consequently, by guiding the signal generated at the detection electrode to a signal processing circuit of the known art, the surface potential can be detected.
The areas around the detection electrode and the stationary electrode are electromagnetically shielded and the stationary electrode is set facing opposite the surface potential measurement target via an unshielded detection window, to enable detection of the surface potential almost completely unaffected by external noise.
It is desirable to vary the value of the impedance between the shield potential and the stationary electrode so as to achieve the shape of a sine wave along the time axis. This makes it possible to obtain a signal essentially achieving a sine-wave shape at the detection electrode.
In the surface potential detection method according to the present invention, the complicated structure necessitated in the prior art in which the electrical field between a detection electrode and the corresponding photosensitive drum is interrupted mechanically by using a tuning fork can be replaced with a single stationary electrode. It is obvious that this method may be effectively adopted to achieve simplification in the structure of the sensor and to facilitate the processes of production, assembly, adjustment, maintenance and inspection.
The surface potential detection apparatus used to implement the surface potential detection method described above includes a surface potential sensor and a signal processing circuit.
The surface potential sensor comprises a shield case, a detection electrode, a stationary electrode and an impedance-varying circuit. The shield case which includes a detection window covers the detection electrode, the stationary electrode and the impedance-varying circuit.
The stationary electrode is provided between the detection window and the detection electrode, and the impedance-varying circuit cyclically changes the impedance between the shield case and the stationary electrode. The signal processing circuit processes the signal output by the surface potential sensor.
It is obvious that the surface potential detection apparatus described above may be directly utilized to implement the surface potential detection method according to the present invention.
The surface potential sensor according to the present invention comprises a shield case, a detection electrode and a stationary electrode. The shield case includes a detection window and the detection electrode is provided inside the shield case. The stationary electrode which is provided inside the shield case is set between the detection window and the detection electrode.
It is obvious that this surface potential sensor may be utilized to constitute the surface potential detection apparatus described earlier.
Other objects, structural features and advantages of the present invention are explained in further detail by referring to the attached drawings. The attached drawings are provided simply to illustrate specific examples.